This invention relates to a cyclonic separation device and more particularly but not solely to a cyclonic separation device for a vacuum cleaner.
Cyclonic separation devices are widely used in vacuum cleaners to separate dirt and dust from the airflow. Typically such vacuum cleaners incorporate a single upstream cyclonic separator which is relatively large in diameter and which is suited to separating heavy dirt and dust particles as well as coarse and fibrous matter from the airflow. Such large diameter cyclonic separators are unable to separate lighter dirt and dust particles and hence a further separation stage is needed downstream of the cyclonic separator.
It is well known to provide vacuum cleaners having a downstream stage which comprises a plurality of smaller diameter cyclonic separators connected fluidly in parallel with each other. Whilst smaller diameter cyclonic separators act to restrict airflow, the number of cyclonic separators is chosen so as not to impede the airflow due to the fact that each cyclonic separator takes a proportion of the airflow from the upstream device.
One such vacuum cleaner is disclosed in GB2490693 in which each cyclonic separator of the second stage comprises a cyclone chamber having a frusto-conical side wall. An air inlet is directed tangentially into the first and widest end of the chamber through the side wall thereof. An air outlet extends axially from an end wall which closes the first end of the cyclone chamber. The second end of the cyclone chamber is open. In some cyclonic separators, the side wall may be parallel or reverse-tapered, although cyclone chambers having a frusto-conical side wall are more suited to separating lighter dust particles.
In use, air enters the cyclone chamber through the inlet and rotates in a vertical manner around the cyclone axis inside the frusto-conical side wall towards the second end of the cyclone chamber. The dust particles in the rotating airflow are forced radially outwardly against the side wall under centrifugal action. The volume of rotating airflow slowly diminishes towards the second end of the cyclone chamber as air is drawn radially inwardly and axially towards the outlet at the first end of the cyclone chamber. However, the dust particles that are forced radially outwardly against the frusto-conical side wall are disposed in a boundary layer and slowly migrate towards the open second end of the cyclone chamber, whereupon they pass out of the cyclone chamber into a collection chamber.
A disadvantage of the above-mentioned arrangement is that dust particles in the boundary layer can become re-entrained in the airflow, particularly if the airflow is heavily laden with dust or if there is a drop in airflow. Also, the speed at which the dust particles migrate is slow and hence the risk of re-entrainment is increased, partly because the volume of dust in the boundary layer is so great that it forms a layer which is too wide to remain inside the boundary layer.
Clearly the separation efficiency can be improved by using a high powered motor to drive the fan, which causes a higher rate of airflow through the cleaner. However, legislation is being introduced which limits the power that vacuum cleaners can consume with the result that the separation efficiency will be detrimental.
We have now devised a cyclonic separation device having an improved efficiency.